1. Field of the Invention
The present invention relates to a liquid crystal compound and a liquid crystal composition containing said liquid crystal compound.
2. Description of the Prior Art
Liquid crystal displays are in widespread use at present. Most of the displays are twisted nematic types which use nematic liquid crystals. However, the nematic liquid crystals have the disadvantage of a long response time of about a few milliseconds.
Some years ago a high speed response liquid crystal display device was proposed which utilizes a ferroelectric liquid crystal compound which gives rise to an optical switching phenomenon.
Ferroelectric liquid crystals were discovered by R. B. Meyer et al (J. Physique, 36, 1-69 (1975)). Moreover, a high speed optical switching phenomenon was discovered for the chiral smectic C phase of ferroelectric liquid crystals by N. A. Clark et al. (Appl. Phys. Lett. 36, 899 (1980)).
The high speed switching of ferroelectric liquid crystals can be utilized in many applications, not only for displays, but also printer heads, light valves, and the like.
For the purposes mentioned above, the compounds which are desired are those which are stable and which show a ferroelectric phase at room temperature with large spontaneous polarization, which is needed for a high speed response at relatively low voltage.
Some smectic phases of ferroelectric liquid crystals are known such as the smectic C, smectic F, smectic H and smectic G, and the like phases.
It is said that the smectic C phase is most suitable for use in displays, because the viscosity of the other phases is so high that a high speed response is not expected.
In Table 1 infra, the compounds which are listed are those which are known as ferroelectric liquid crystal compounds. In Table 1, C stands for a crystal phase, SH* for a chiral smectic H phase, SC* for a chiral smectic C phase, SG* for a chiral smectic G phase, SF* for a chiral smectic F phase, SA for a smectic A phase, Ch for a cholesteric phase, and I for an isotropic phase.
Some of the ferroelectric liquid crystal compounds shown, i.e. Compound No. 1 and 2, have a Shiff base (--CH.dbd.N--) structure, which means that they are easily hydrolyzed, and accordingly, the compounds are not stable. On the other hand, Compounds 3 and 4 don't have a Shiff-base structure, but they have small spontaneous polarization. The values of spontaneous polarization are less than 10 nC/cm.sup.2. Still further Compounds 5-7 show a smectic F phase, which phase is highly viscous and is an unsuitable phase for the display because of the rather slow switching speed of the compounds. Moreover, the temperature range of the smectic F phase of these compounds is far from room temperature. A need therefore continues to exist for liquid crystal compounds which are stable, which have large spontaneous polarizations and which have satisfactory temperature ranges for displays.